1. Field of the Invention
The present invention relates to a controller for an induction motor and a control method for an induction motor, and especially relates to a speed controller for an induction motor and a speed control method for an induction motor.
2. Description of Related Art
A control method for an induction motor is known in which a sine wave is generated by using a microcomputer and an inverter supplies the induction motor with electric power based on the sine wave. A speed of the induction motor is changed by changing a frequency of the sine wave.
FIG. 1 shows a speed controller for a single-phase induction motor disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 6-213912). A signal inputting section 59 outputs a speed command signal based on a user's operation. A microcomputer 53 outputs digital data indicating sine wave values stored in a ROM table 500 based on the speed command signal. A digital to analog (D/A) converter 52 converts the digital data indicating the sine wave values into a sine wave signal. A triangle wave generator 51 generates a triangle wave signal. A comparator 50 outputs a control signal based on a comparison between the sine wave signal and the triangle wave signal. An inverter 57 supplies alternating current power to a motor 58 based on the control signal. The motor 58 rotates at a rotation speed corresponding to a frequency of the sine wave signal.
The ROM table 500 stores digital data of a sine wave for one period in addresses 0 to 100. As shown in FIG. 2, the addresses 0 to 25 store 80h to FFh as digital data of the sine wave for a quarter of the period, the addresses 25 to 50 store FFh to 80h as digital data for a next quarter of the period, the addresses 50 to 75 store 80h to 00h as digital data for a next quarter of the period, and the addresses 75 to 100 store 00h to 80h as digital data for a next quarter of the period. The symbol “h” indicates a hexadecimal notation.
Japanese Laid Open Patent Application (JP-A-Heisei 5-336791) discloses a servo device which reduces a storage capacity for storing sine wave data by calculating sine wave values and cosine wave values from the sine wave data for (2N+1)/2 period.
The present inventor has recognized that there is a room to reduce data amount of sine wave data used for the control of an induction motor.